1. Field of the Invention
The present invention relates to a method for cleaning an out-of-basin type separation membrane for filtering tank water of a biological treatment tank with circulation.
2. Description of the Related Art
A biological treatment tank for decomposing organic substances and for nitrification and denitrification taking advantage of actions of microorganisms is widely used for the treatment of sewage, side streams, industrial waste water, leachate, night soil, agricultural waste water, live-stock waste water and fish-breeding waste water or the like. Tank water in the biological treatment tank has been usually guided to a final sedimentation basin for solid-liquid separation by gravitational sedimentation, and sludge and supernatant are usually separated there with each other. However, since gravitational sedimentation requires a long retention time, a wide footprint is needed for the sedimentation basin. Accordingly, a separation membrane is often used as a solid-liquid separation means in sewage treatment plants constructed in urban areas where the site area is limited.
While the separation membrane is classified into an out-of-basin type and a submerged type, cleaning of the sludge adhered on the membrane surface is necessary in any of these two types. In particular, tank water in the biological treatment tank often contains a large amount of MLSS (Mixed Liquor Suspended Solid), the membrane is quickly plugged without appropriate cleaning of the membrane surface.
As disclosed in Japanese Patent Application Laid-Open (JP-A) No. 9-136021, the membrane surface of the submerged type separation membrane is continuously cleaned by introducing solid particles into the tank, allowing the surface of the separation membrane to contact with the solid particles by forming a circulating flow with air bubbles, and scratching away the sludge adhered on the surface of the membrane. However, since such circulation flow with air bubbles cannot be utilized in the out-of-basin type separation membrane, it is usually cleaned by backwash cleaning and chemical cleaning as is used in conventional separation membranes.
Usually, the separation membrane of the out-of-basin type is connected to the biological treatment tank through a circulation passageway, and tank water is filtered by a cross-flow method. Consequently, a raw water side of the separation membrane is connected to the biological treatment tank via the circulation passageway, and the sludge caused by allowing deposits to be peeled to the raw water side by applying a backwash pressure from a filtrate side is returned to the biological treatment tank as backwash drain through the backwash drain circulation passageway, or is returned to the biological treatment tank as backwash drain through the backwash drain circulation passageway together with the sludge remaining in the raw water side of the membrane after peeling. Since this is an operation necessary for maintaining MLSS (mixed liquor suspended solid) constant in the biological treatment tank, biological treatment ability of the biological treatment tank may be impaired when the backwash drain containing the peeled sludge is not returned.
So far, the peeled sludge has been returned to the biological treatment tank when the separation membrane is subjected to chemical cleaning as in backwash cleaning with water mentioned above. However, different from conventional separation membranes used for purifying water by filtration, a large amount of the sludge is deposited on the membrane surface of the out-of-basin type separation membrane for filtering tank water in the biological treatment tank by circulation. Accordingly, a large amount of chemicals are required for dissolving the sludge by conventional methods in which the separation membrane is immersed in a solution of chemicals, and backwash water containing a large amount of chemicals is returned to the biological treatment tank through the backwash water draining passageway or circulation passageway. Consequently, microorganisms in the biological treatment tank are damaged by the flowing-in chemical solution. A long time is required for resuming biological activity, and membrane filtration performance and treatment ability of the biological treatment tank may be deteriorated during the recovery time.